Rename default branch to main

This is the only place I did find main in the code.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

README.md

@@ -1,5 +1,5 @@
 # Common
-[![circleci](https://circleci.com/gh/prometheus/common/tree/master.svg?style=shield)][circleci]
+![circleci](https://circleci.com/gh/prometheus/common/tree/main.svg?style=shield)
 
 This repository contains Go libraries that are shared across Prometheus
 components and libraries. They are considered internal to Prometheus, without